Wavelength-Agile External-Cavity Diode Laser for DWDM

A prototype external-cavity diode laser (ECDL) has been developed for communication systems utilizing dense wavelength- division multiplexing (DWDM). This ECDL is an updated version of the ECDL reported in Wavelength-Agile External- Cavity Diode Laser (LEW-17090), NASA Tech Briefs, Vol. 25, No. 11 (November 2001), page 14a. To recapitulate: The wavelength-agile ECDL combines the stability of an external-cavity laser with the wavelength agility of a diode laser. Wavelength is modulated by modulating the injection current of the diode-laser gain element. The external cavity is a Littman-Metcalf resonator, in which the zeroth-order output from a diffraction grating is used as the laser output and the first-order-diffracted light is retro-reflected by a cavity feedback mirror, which establishes one end of the resonator. The other end of the resonator is the output surface of a Fabry-Perot resonator that constitutes the diode-laser gain element. Wavelength is selected by choosing the angle of the diffracted return beam, as determined by position of the feedback mirror. The present wavelength-agile ECDL is distinguished by design details that enable coverage of all 60 channels, separated by 100-GHz frequency intervals, that are specified in DWDM standards

Identification and Quantitative Measurements of Chemical Species by Mass Spectrometry

The development of a miniature gas chromatograph/mass spectrometer system for the measurement of chemical species of interest to combustion is described. The completed system is a fully-contained, automated instrument consisting of a sampling inlet, a small-scale gas chromatograph, a miniature, quadrupole mass spectrometer, vacuum pumps, and software. A pair of computer-driven valves controls the gas sampling and introduction to the chromatographic column. The column has a stainless steel exterior and a silica interior, and contains an adsorbent of that is used to separate organic species. The detection system is based on a quadrupole mass spectrometer consisting of a micropole array, electrometer, and a computer interface. The vacuum system has two miniature pumps to maintain the low pressure needed for the mass spectrometer. A laptop computer uses custom software to control the entire system and collect the data. In a laboratory demonstration, the system separated calibration mixtures containing 1000 ppm of alkanes and alkenes

Infrared photochemistry of ethylene clusters

Infrared irradiation of ethylene clusters formed in supersonic molecular beams, using a low power cw CO2 laser, results in the photodissociation of a large fraction of the van der Waals molecules. Under such conditions, infrared absorption intensity exhibits first-order power dependence and is readily detected as loss in molecular beam intensity. Intramolecular energy transfer rates, determined by measuring spectral linewidths, are shown to vary with the vibrational mode initially excited. Ethylene clusters containing one quantum of vibrational energy corresponding to the nun7 fundamental in the monomer (949 cm^–1) have a vibrationally predissociative lifetime of 0.33 psec. In comparison, the relaxation rate of ethylene-d4 clusters with one quantum of excitation corresponding to the nu12 (1078 cm^–1) mode of C2D4 is 0.7 psec

Field assessment of sensor technology for environmental monitoring using a process-based soil gas method at geologic CO2 storage sites

Bureau of Economic Geolog

PSEUDO-RANDOM AMPLITUDE OR TONE BURST MODULATION COMBINED WITH CAVITY-ENHANCED DETECTION

Author Institution: Southwest Sciences Ohio Operations, Cincinnati OH 45244 ; Southwest Sciences, Santa Fe NM 87505Combining pseudo random-modulation techniques with cavity enhanced spectroscopy allows the estimation of both optical path length and absorption while using a cw diode laser as the light source. Two pseudo-random modulation approaches are described, in each case combined with an off-axis cavity consisting of two cylindrical mirrors oriented to produce a dense pattern of cavity modes. In one approach, the amplitude of the laser is modulated; deconvolution results in an exponential ring-down decay curve. In the second approach, tone bursts that modulate the laser frequency are switched on and off with a pseudo-random code. Deconvolution yields the first derivative of an exponential whose amplitude is proportional to absorbance. With either approach the dynamic range of the transmitted intensity is much smaller than for a ring-down experiment, allowing high gain before the signal is digitized. Initial experimental results measuring carbon monoxide are presented, and the dependence of detection sensitivity on experimental parameters such as pseudo-random clock rate is discussed

Statistical Characterization of Temperature and Pressure Vertical Profiles for the Analysis of Laser Heterodyne Radiometry Data

The statistical analysis of historic pressure and temperature profiles from radiosonde launches for use in the fitting of molecular oxygen line shapes is presented. As the O2 mixing ratio is nearly constant throughout the lower atmosphere, only variations in pressure and temperature profiles will affect the fit of observed O2 features in Laser Heterodyne Radiometry (LHR) spectra. Radiosonde temperature and pressure data are extracted from the Integrated Global Radiosonde Archive (IGRA) for a given station, date, and launch time. Data may be extracted for a single launch, for the same date over several years, and/or within a window centered on a target date. The temperature and pressure profiles are further characterized by the statistical variation in coefficients of polynomial fits in altitude. The properties of the probability distributions for each coefficient are used to constrain fits of O2 line shapes through Nelder–Mead optimization. The refined temperature and pressure profiles are then used in the retrieval of vertically resolved mixing ratios for greenhouse gases (GHGs) measured in the same instrument. In continuous collections, each vertical profile determination may be treated as a Bayesian prior to inform subsequent measurements and provide an estimate of uncertainties